1. Field of the Invention
The present invention relates to a reciprocating (linear) motor compressor and, more particularly, to an apparatus and method for controlling an operation of a reciprocating motor compressor.
2. Description of the Prior Art
In general, a reciprocating motor compressor (hereinafter, for simplicity, referred to as “reciprocating motor compressor”) which is operated by a linearly reciprocating electric motor without a crank shaft for converting a rotational motion to a linear motion, a reciprocating compressor has less friction loss, and thus, can provide a higher compression efficiency than other compressors.
When the reciprocating motor compressor is used for a refrigerator or an air-conditioner, a compression ratio of the compressor can be varied to control the cooling capacity by varying a stroke voltage applied to the reciprocating motor of the compressor.
The conventional controlling of a reciprocating motor compressor will now be described with reference to FIG. 1.
FIG. 1 is a block diagram showing the construction of an apparatus for controlling an operation of a reciprocating motor compressor in accordance with the prior art.
As shown in FIG. 1, the apparatus for controlling an operation of a reciprocating motor compressor includes a current detector 4 detecting a current applied to the reciprocating motor (not shown) of the compressor 6, a voltage detector 3 detecting a voltage applied to the motor, a stroke calculator 5 calculating a stroke estimation value based on the detected current and voltage values and a certain operational parameter of the motor, a comparator 1 comparing the calculated stroke estimation value with a pre-set stroke reference value, and outputting a difference vale according to the compared result, and a stroke controller 2 controlling an operation (stroke) of the compressor 6 by varying the voltage applied to the motor by controlling a turn-on period of a triac (not shown) connected in series with the motor according to the difference value. Herein, when the stroke controller 2 controls the turn-on period of the triac applying power to the motor, it controls a firing angle (α1) of the triac during of a positive (+) phase and a firing angle (α2) of the triac during of a negative (−) phase of the current flowing at the triac to be the same, i.e., α1=α2.
The operation of the apparatus for controlling the operation of the reciprocating motor compressor will now be described with reference to FIG. 1.
First, the current detector 4 detects the current being applied to the motor (not shown) of the compressor 6 and outputs the value of the detected current to the stroke calculator 5. At this time, the voltage detector 3 detects the voltage being applied to the motor and outputs the value of the detected voltage to the stroke calculator 5.
The stroke calculator 5 calculates the stroke estimation value (X) of the compressor based on the detected current and voltage values and the operational parameter of the motor according to equation (1) shown below, and then applies the calculated stroke estimation value (X) to the comparator 1:
                    X        =                              1            α                    ⁢                      ∫                                          (                                                      V                    M                                    -                  Ri                  -                                      L                    ⁢                                          i                      _                                                                      )                            ⁢                              ⅆ                t                                                                        (        1        )            wherein ‘R’ is the motor resistance value, ‘L’ is the motor inductance value, ‘α’ is a motor constant, Vm is the value of the voltage applied to the motor, ‘I’ is the value of the current applied to the motor, and ī is the differentiated value (di/dt) of ‘i’.
Thereafter, the comparator 1 compares the stroke estimation value and the stroke reference value, and applies a difference value according to the compared result to the stroke controller 2.
The stroke controller 2 controls the stroke of the compressor 6 by varying the voltage being applied to the motor of the compressor 6 based on the difference value. This will be described with reference to FIG. 2 as follows.
FIG. 2 is a flow chart of a method for controlling an operation of the reciprocating motor compressor in accordance with the prior art.
First, when the stroke estimation value is applied to the comparator 1 by the stroke calculator 5 (step S1), the comparator 1 compares the stroke estimation value with the pre-set stroke reference value (step S2) and outputs the difference value according to the compared result to the stroke controller 2.
If the stroke estimation value is smaller than the stroke reference value, the stroke controller 2 increases a voltage applied to the motor in order to control the stroke of the compressor (step S3). If, however, the stroke estimation value is greater than the reference value, the stroke controller 2 reduces the voltage applied to the motor (step S4). In this case, in order to increase or reduce the voltage applied to the motor, the turn-on period of the triac (not shown) electrically connected with the motor is controlled to apply the voltage to the motor.
Meanwhile, when the motor is controlled through the triac, a loss in the motor increases. The reason for this will now be described with reference to FIGS. 3A and 3B.
FIGS. 3A and 3B are waveform diagrams showing waveforms of the voltage and current applied to the motor of the reciprocating motor compressor in accordance with the prior art.
As shown in FIG. 3A, in the apparatus for controlling an operation of the reciprocating motor compressor, when the turn-on period of the triac applying power to the motor is controlled, the firing angle (α1) of the triac during the positive (+) half cycle phase of the voltage flowing at the triac and the firing angle (α2) of the triac during the negative (−) half cycle phase are controlled to be the same. Consequently, the positive phase and the negative phase of the current applied to the motor become relatively asymmetrical.
With reference to FIG. 3B, the asymmetrical current can be divided into symmetrical AC component current and a DC component current, and the DC component current increases a loss in the motor. Namely, the loss in the motor increases according to equation (2) shown below:Motorloss=i2(RAC+RDC)   (2)wherein ‘i’ is the current applied to the motor, ‘RAC’ is an iron loss and a copper loss arising due to the AC component current applied to the motor, and ‘RDC’ is a copper loss arising due to the DC component current applied to the motor. It is noted that the AC component contributes to both the iron loss and the copper loss, while the DC component contributes only to the copper loss.
As stated above, the conventional apparatus for controlling an operation of the reciprocating motor compressor has the problem that since the DC component current is applied, the loss in the motor increases.
U.S. Pat. No. 6,779,982 issued on Aug. 24, 2004 also discloses a conventional reciprocating motor compressor.